project: fallback to reading HEAD when rev-parse fails

git rev-parse fails on invalid HEAD, e.g. after incomplete sync, causing
NoManifestException. Fall back to v2.56's direct file reading when
rev-parse fails.

Bug: 435045466
Change-Id: Ia14560335110c00d80408b2a93595a84446f8a57
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/495181
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Scott Lee <ddoman@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

project.py

diff --git a/project.py b/project.py
index 4c699d4..2badfb0 100644
--- a/project.py
+++ b/project.py
@@ -3841,8 +3841,29 @@
                     return self.rev_parse(HEAD)
                 return symbolic_head
             except GitError as e:
+                logger.warning(
+                    "project %s: unparseable HEAD; trying to recover.\n"
+                    "Check that HEAD ref in .git/HEAD is valid. The error "
+                    "was: %s",
+                    self._project.RelPath(local=False),
+                    e,
+                )
+
+                # Fallback to direct file reading for compatibility with broken
+                # repos, e.g. if HEAD points to an unborn branch.
                 path = self.GetDotgitPath(subpath=HEAD)
-                raise NoManifestException(path, str(e))
+                try:
+                    with open(path) as fd:
+                        line = fd.readline()
+                except OSError:
+                    raise NoManifestException(path, str(e))
+                try:
+                    line = line.decode()
+                except AttributeError:
+                    pass
+                if line.startswith("ref: "):
+                    return line[5:-1]
+                return line[:-1]
 
         def SetHead(self, ref, message=None):
             cmdv = []